Visitors tour the John F. Kennedy Hyannis Museum in Hyannis, Mass., on Nov. 14. A new poll shows that conspiracy theories about who is responsible for the Nov. 22, 1963, assassination are still topics of fascination for Americans, across partisan lines.

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WASHINGTON — Fifty years after the assassination of John F. Kennedy, many Americans remain unconvinced by the official explanation of his killing. The Warren Commission, headed by US Supreme Court Chief Justice Earl Warren, concluded that Lee Harvey Oswald acted alone. But today, 61 percent of respondents to a new Gallup poll still say they believe someone other than Oswald was involved.

The nation is split on who this someone might be. Thirteen percent blame organized crime, according to the just-released Gallup survey. Thirteen percent believe the culprit might be a conspiracy within the federal government.

Seven percent point a finger at the CIA and five percent at Fidel Castro. Fully 40 percent say they’re not certain who else was involved.

The belief in a JFK conspiracy has waned in recent years, dropping from a high of 81 percent of Americans in 2000. But given the unanswered questions about his tragic death, such as how Oswald could have fired three bullets in a way consistent with Kennedy’s wounds, it is unlikely the issue will soon be put to rest, writes Gallup’s Art Swift.

“Speculating about who was really responsible for Kennedy’s death will likely remain a topic of fascination for the American public for many years to come,” writes Mr. Swift.

The circumstances of Oswald’s death are surely a main reason conspiracy theories persist. He was shot by Dallas night club owner Jack Ruby two days after the assassination, before being thoroughly questioned by federal agents.

There are also demonstrable holes in the Warren Commission’s case. Its investigation was rushed and some key witnesses weren’t called to testify. While the vast majority of government documents pertaining to the assassination have been released, a large number remain classified and hidden.

Reputable estimates put the number of unreleased CIA documents alone at about 1,171, according to University of Virginia professor of political science Larry Sabato, author of “The Kennedy Half-Century: The Presidency, Assassination and Lasting Legacy of John F. Kennedy."

“Even a half-century later, we don’t have the complete story of the assassination,” wrote Mr. Sabato in a Washington Post opinion piece.

Conspiracy theories about Kennedy’s death also have a unique appeal to a wide range of Americans, as opposed to conspiracy theories about the Sept. 11 terrorist attacks, say, or the persistent falsehood that President Obama was born in Kenya.

Many conspiracy theories are perpetuated by partisan beliefs, according to University of Miami political scientists Joseph Uscinski and Joseph Parent, authors of an upcoming book on the subject. Conspiracy theorists who think that dark forces pulled the US into the war in Iraq tend to be of a liberal bent. Obama "birthers" tend to be conservative.

Because of this inherent partisan lean, most US conspiracy theories can attract only a quarter to a third of the US population at best, write Mr. Uscinksi and Mr. Parent in the "Monkey Cage" political science blog.

But JFK conspiracy theories cut across partisan lines, as there are a wide array of possible villains, from an alleged hard-line conspiracy of security officials who thought Kennedy soft on the Soviets and Vietnam, to communist Castro and the Soviet Union itself.

“Kennedy conspiracy theories are significantly more popular.... In fact, more Americans believe that a shadowy conspiracy was behind a president’s death 50 years ago than know who Joe Biden his,” conclude Uscinksi and Parent.